The aim of the proposed research is to understand the mechanisms by which IgD expression is regulated in B lymphocytes that are at various stages of their differentiation. Pre B lymphocytes, immature B lymphocytes and activated B lymphocytes express little or no membrane IgD, whereas mature, but resting, adult B lymphocytes express easily detectable amounts. As a first step toward understanding these changes in IgD expression, normal B lymphocytes at the different stages of differentiation will be studied. Radioimmunoassays will be used to quantitate both membrane and total cellular IgD and IgM in purified B lymphocyte populations and Northern blot analyses will be used to determine the amount of Delta and Mu mRNA in the same cells. The mechanisms regulating IgD expression will be studied in more detail in cell lines whose expression of Delta and IgD closely resemble the expression patterns observed in pre B and immature B cells, mature B lymphocytes, and activated B lymphocytes. Regulation at the level of transcription and posttranscription will be studied by determining the length of the Mu-Delta locus that is transcribed in the three types of cells, by determining the role of polyadenylation in determining whether Mu or Delta is expressed, and by characterizing partially processed RNA species. Regulation at the level of translation or posttranslation will be studied by in vitro translation experiments and by biosynthetic pulse-chase labelling. A more detailed understanding of the expression and regulation of IgD will provide a better basis on which to evaluate its role in the immune response.